1. Field of the Invention
The present invention is generally related to the evaluation of semiconductor material, particularly evaluating the bulk properties of the semiconductor material.
2. Description of the Related Technology
In semiconductor processing the properties of semiconductor materials, such as Si, SiGe, GaAs, etc. . . . , and their dependence on processing conditions need to be determined. The properties of the bulk material can be changed by introducing species, e.g., by ion implantation, by annealing, e.g., Rapid Thermal processing (RTP), by manufacturing of the substrate, etc. In CMOS (Complementary Metal Oxide Silicon) devices, for example, the junction depth and profile of the source/drain regions formed in the semiconductor substrate need to be determined. To yield advanced, high-performance Complementary Metal-Oxide-Semiconductor (CMOS) technologies, it is crucial to be able to characterize quickly and reliably ultra shallow junctions. The process conditions can then be optimized to obtain the desired junction depth and profile and hence the required device characteristics.
There exist various methods to investigate the properties of the semiconductor profile. Some of these techniques are destructive, for example, spreading-resistance-profile (SRP) whereby the semiconductor substrate is cleaved along a diagonal cleavage line and a two point electrical measurement is performed at subsequent positions along this cleavage line. Some techniques are non-destructive, for example, the Carrier Illumination (CI) technique, as disclosed in U.S. Pat. Nos. 6,049,220 and 6,323,951, both hereby incorporated by reference in their entirety. For in-line monitoring of the pre- and post-anneal process steps, this Carrier Illumination™ technique has established itself as a fast, non-contact, non-destructive tool with wafer mapping capability. For process monitoring applications, the exact quantitative interpretation of the CI signal is less important as long as high repeatability and sensitivity for a particular profile or process parameter can be demonstrated.
In CI, typically, a focused “pump” laser beam (also labeled generation beam), operating at a fixed wavelength of typically 930 nm, which is larger than the band gap of the material under study, is used to generate a quasi-static excess carrier profile in the bulk of the semiconductor profile, giving rise to a depth dependent index of refraction. The excess carriers distribute themselves in the semiconductor material according to a profile which is defined as the carrier concentration (in number of carriers per cubic cm exceeding the level of carriers present within the semiconductor substrate without stimulation labeled as the background carrier concentration or profile), e.g., in the absence of illumination. This background concentration is dependent, inter alia, on the concentration of dopant atoms. Specifically, the excess carrier concentration changes from being zero outside a surface of the semiconductor material to a finite value inside the semiconductor material. This change results in a step increase in the concentration of excess carriers at the surface of the semiconductor substrate. This step increase of the excess carriers concentration at the interface between the semiconductor material under study and its surroundings, e.g., air, will be labeled as the near-surface component which will result in a near-surface component of the reflected probe beam as will be discussed later on. As the depth z, defined from the front surface into the semiconductor substrate, increases, the excess carrier concentration changes in a manner proportional, inter alia, to the change in the concentration of dopant atoms or to the presence of recombination centers. For example, in some cases, the dopant concentration rises, but in other cases the dopant concentration dips first and then rises, depending on the detailed shape of the doping profile.
The measured CI-signal is then generated by illuminating the semiconductor material with a second “probe” laser (also labeled analyzer beam), having a fixed wavelength higher than the fixed wavelength of the “pump” laser, typically 990 nm. This probe laser will be reflected at the sample surface and/or at any region with a large change in the index of refraction.